This invention relates to a liquid crystal display device of battery drive type.
Liquid crystal display devices are widely used in the display section of desk computers and various measuring instruments. For example, a liquid crystal display device is used in a digital electronic thermometer which senses the body temperature by a temperature sensor such as a thermistor and converts the output of the sensor into a corresponding digital signal to display the body temperature. In this case, the liquid crystal device is employed for the purpose of reducing the power consumption in display section of the battery used as driving power source for the digital electronic thermometer.
In the digital electronic thermometer of this kind, unlike desk computers or the like which deal with only digital quantities, the body temperature, which is an analog quantity, is detected by a temperature sensor, and the detected analog quantity is converted in an operation circuit into a digital quantity for displaying the measured body temperature in the digital form. In this case, if the output voltage of a power supply battery employed is reduced to a certain extent, the analog-digital conversion in the operation circuit can be erroneously affected although the display device operates normally. This results in an erroneous measurement information display on the display section. Accordingly, it is necessary to inform the user that the electromotive force of the power supply battery is reduced lower than a predetermined value and also the appropriate time when the battery has to be replaced. With the reduction of the battery voltage, the difference in brightness between the liquid crystal display portion and the background portion of the display portion, that is, the contrast between these portions, is reduced, and hitherto it has been in practice to replace the battery when the displayed content can no longer be clearly seen. However, the contrast between the liquid crystal display section and surrounding portion is greatly influenced by the angle at which the display section is observed, and therefore, with the above method it is difficult to determine the accurate time when the battery has to be replaced.